In the original mythology, a succubus turning into an incubus and vice-versa was normal, accepting the semen from males to change its genetic makeup in their own bodies to impregnate females and reproduce more succubus, incubus, or some other demon. All of this is accomplished with seductive/charming powers to make their partners more willing to copulate with them, and I've read they were originally hideous in appearance.
Kind of weird they're dropping the supernatural/fantasy bombshell now at this time, but it kind of follows what the original myth is portrayed, so I'm down with it.